Fischer–Tropsch Synthesis: Kinetics and Water Effect on Methane Formation over 25%Co/γ-Al2O3 Catalyst

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• 作者：Wenping Ma ; Gary Jacobs ; Tapan K. Das ; Cornelius Mduduzi Masuku ; Jungshik Kang ; Venkat Ramana Rao Pendyala ; Burtron H. Davis ; Jennifer L. S. Klettlinger ; Chia H. Yen
• 刊名：Industrial & Engineering Chemistry Research
• 出版年：2014
• 出版时间：February 12, 2014
• 年：2014
• 卷：53
• 期：6
• 页码：2157-2166
• 全文大小：359K
• ISSN：1520-5045

文摘

The kinetics and the effect of indigenous and externally added water on methane formation during Fischer鈥揟ropsch synthesis (FTS) was studied over Co based catalysts using a 1 L continuously stirred tank reactor (CSTR). The water cofeeding study (10% water) was conducted over a 0.27%Ru鈥?5%Co/Al₂O₃ catalyst at a low CO conversion level of 19% at 220 掳C in order to lessen the effect of catalyst aging during the addition of water, while the kinetic experiment was conducted over 25%Co/纬-Al₂O₃ at the conditions of 205鈥?30 掳C, 1.4鈥?.5 MPa, H₂/CO = 1.0鈥?.5, and 3鈥?6 (NL/g_cat)/h (X_CO < 60%). Indigenous and externally added water decreases methane formation by a kinetic effect. The addition of 10% water led to a decrease in the CH₄ rate by 12% (3.5 鈫?3.0 (mmol/g_cat)/h), while little catalyst deactivation was observed during water addition. Increases in indigenous water partial pressure also lowered the CH₄ rate and its selectivity. Kinetic analysis was performed using a group of 220 掳C data collected between 365 and 918 h when the deactivation rate was very low. An empirical CH₄ kinetic model, with a water effect term (P_H2O/P_H2), (r_CH4 = kP_CO ^aP_H2^b/(1 + mP_H2O/P_H2)) was used to fit kinetic data. The CH₄ kinetic results suggest a negative water effect on CH₄ formation during FTS on the unpromoted cobalt catalyst, consistent with the water effect results. The final methane kinetics (r_CH4) equation obtained at 220 掳C over 25%Co/纬-Al₂O₃ is as follows: r_CH4/[(mol/g_cat)/h] = 0.001053{P_CO^鈥?.86P_H2^1.32/[1 + 0.46(P_H2O/P_H2)]}. Meanwhile, a methane selectivity model at 220 掳C for the 25%Co/Al₂O₃ catalyst was also developed: S_CH4 = 0.0792P_CO^鈥?.55P_H2^0.44[(1 鈥?0.24P_H2O/P_H2)/(1 + 0.46P_H2O/P_H2)]. The CH₄ selectivity model provided a good prediction of CH₄ selectivities under the experimental conditions used. Furthermore, our empirical CH₄ kinetic results on the cobalt catalyst are consistent with literature kinetic models that were derived from carbide mechanisms; high CH₄ selectivity from the cobalt catalyst is found to be mainly due to a high CH₄ reaction rate constant.